Abstract: Aiming at the intense boiling and condensation interaction in the narrow space inside the flat heat pipe spreader, taking deionized water as the working fluid, the condensation phenomenon on the condensing surface and its influence on the heat transfer performance of the phase change chamber were studied by visual experiment. The internal mechanism that the condensing phenomenon affects heat and mass transfer characteristics in boiling-condensation coexistence chamber under different heat flux and liquid filling ratios was revealed. The formation, growth, convergence of liquid film, and dripping of droplets on the condensation surface were analyzed, as well as the scouring of boiling bubbles generated at the gas-liquid interface with the movement of fluid flow on the condensed liquid film. Experimental results show that under the influence of gravity, water vapor condensing on the condensation surface represents the characteristics of liquid droplets and liquid film coexisting and transforming into each other. The thermal resistance of the condensation surface accounts for 65%-88% of the total thermal resistance of phase change chamber. Appropriate liquid filling ratio is beneficial to washing the condensing wall surface by the boiling bubbles, accelerating the reflux of the condensing liquid, and improving the heat transfer performance of the condensing surface and phase change chamber. With the increase of heat flux, the boiling process becomes more intense, the frequency of scouring the condensing surface by the boiling bubbles increases, the reflux speed of the condensate increases, the thermal resistance of the condensing surface decreases, and the heat transfer performance is improved.